[WIP] Foxx.Repository should provide more methods

|ArangoDB Collection|Foxx.Repository|
|:--|:--|
| `save` | `save` |
| `document` | `byId` |
| `byExample` | `byExample` |
| `firstExample` | `firstExample` |
| `all` | **New:** `all` |
| *Remove providing a model* | **New:** `remove` |
| `remove` | `removeById` |
| `removeByExample` | `removeByExample` |
| *Replace providing a model* | `replace` |
| `replace` | `replaceById` |
| `replaceByExample` | **New:** `replaceByExample` |
| `update` | **New:** `updateById` |
| `updateByExample` | **New:** `updateByExample` |
| `count` | **New:** `count` | 

(There are also `first`, `last`, `any` and `toArray` – I will not add them. `range` is only possible if there is a skiplist index present on that field, so I won't add it for all collections. See #899)